CHEMISTRY · Multi-part · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends
Problem
Oxygen (O, Z = 8), sulfur (S, Z = 16), and selenium (Se, Z = 34) belong to group 16. Selected enthalpy data for adding electrons are listed below.
• First electron affinity (EA₁):
– O: −141 kJ·mol⁻¹ S: −200 kJ·mol⁻¹ Se: −195 kJ·mol⁻¹
• Second electron affinity (EA₂) for O only: +744 kJ·mol⁻¹ (positive values indicate energy input is required)
No experimental EA₂ values are provided for S or Se.
(a) Explain why EA₁ becomes less exothermic moving from S to Se, even though nuclear charge increases.
(b) Discuss the key energetic factors that make EA₂ for O positive (endothermic).
(c) Predict whether EA₂ for S is likely to be greater than, less than, or approximately equal to EA₂ for O. Support your prediction with a detailed comparison of nuclear charge, atomic radius, and electron–electron repulsion.
(d) Evaluate the statement: "A higher nuclear charge always leads to a more negative (more exothermic) electron affinity." Use evidence from group-16 data and your reasoning from parts (a) through (c) to defend or refute the claim.